Best way to complete a cylinder from a segment
Use Extrusion and the Mirror Modifier

Extrusion

If it is just a segment, first Tab into edit mode, rotate to front view (Numpad 1), key E, then Z, and drag the mouse downwards to the desired height for the cylinder.

Mirror Modifier

With the cylinder selected, go to the Object Modifiers tab in the Properties panel and add a Mirror modifier. Set the settings as follows:



This will reflect the segment about the origin (the orange point with the green and red arrows coming from it).